Secondary cells for use in, e.g., portable electronic devices, hybrid vehicles and electric vehicles have been developed, in particular, lithium-ion secondary cells are widely known. In the circumstances, lithium-containing olivine-type metal phosphate such as Li(Fe,Mn)PO4, which are compounds not greatly influenced by resource limitation and exhibit high safety, is suitably used as positive electrode materials for obtaining high-power and large-capacity lithium secondary cells. However, these compounds have characteristics derived from crystal structures, based on which it is difficult to sufficiently enhance conductivity. In addition, there is still room for improvement in diffusivity of a lithium ion. Accordingly, various developments have conventionally been made.
For example, in Patent Literature 1, the lithium ion diffusion distance in an olivine-type positive electrode active material is shortened by forming a primary crystal particle as an ultrafine particle, to be tried to improve performance of the electric cell to be obtained. In Patent Literature 2, a conductive carbonaceous material is uniformly deposited onto the surface of a particle of a positive electrode active material to obtain a regular electric field distribution on the particle surface. In this manner, high power generation of the electric cell is tried to attain.
In the case where the surface of a particle of a positive electrode active material is coated with carbon, the amount of lithium atoms passing through the carbon film is limited, with the result that it is difficult to enhance charge/discharge characteristics against intention. To avoid this phenomenon, Patent Literature 3 discloses a method of coating the surface of a particle of a positive electrode active material with a carbon nanostructure such as a carbon nanotube and nano-graphene by plasma decomposition of an organic compound.
Patent Literature 4 discloses, as a technique for improving physical properties of an electric cell by enhancing the binding property between an electrode active material and a conductive aid, a slurry composition for electrode, containing an electrode active material, a conductive aid and a cellulose fiber as an aqueous binder. The slurry is applied onto an electrode current collector and dried to form an electrode active material layer.
Since lithium is a rare and valuable substance, investigation on e.g., a sodium-ion secondary cell using sodium in place of a lithium-ion secondary cell has been started.
For example, Patent Literature 5 discloses a positive-electrode active material for a sodium secondary cell using malysite-type NaMnPO4, and Patent Literature 6 discloses a positive-electrode active material containing a transition metal doped sodium phosphate having an olivine structure. Either one of the literatures discloses that a high-performance sodium-ion secondary cell can be obtained.